Conventional optical waveguide formation materials for hybrid flexible printed wiring boards for optical/electrical transmission employ photosensitive epoxy resin compositions. Where a core layer is formed in a predetermined pattern by using any of the photosensitive epoxy resin compositions, for example, the formation of the predetermined core pattern is achieved by irradiation with ultraviolet radiation (UV) via a photomask.
Such a photosensitive epoxy resin composition has high photocurability, but is disadvantageous in that the photosensitive epoxy resin composition cannot be employed for a continuous process such as an R-to-R (roll-to-roll) process because of the surface tackiness of a coating film of the photosensitive epoxy resin composition (the film of the photosensitive epoxy resin composition is liable to be broken when being brought into contact with a roll) and, therefore, the productivity is poorer (PTL 1). Therefore, a resin component that is solid at room temperature is generally used as a photosensitive resin for R-to-R adaptability. As the molecular weight of the resin component becomes higher, the flexibility of an uncured amorphous film of the resulting resin composition is increased, but the patterning resolution is reduced. As the molecular weight of the resin component becomes lower, on the other hand, the patterning resolution is increased, but the flexibility of the resulting resin composition is reduced. Problematically, the flexibility and the patterning resolution of the film are in tradeoff relation. Therefore, there is a demand for an optical waveguide material which satisfies the requirements for both the flexibility and the patterning resolution of the film. For example, a resin composition containing an epoxy-containing acryl rubber, urethane (meth)acrylate or a (meth)acrylate free from a urethane bond is proposed as a cladding layer forming material for an optical waveguide (PTL 2).
Incidentally, the optical waveguide core layer forming material needs to satisfy requirements for various physical properties such as higher refractive index, higher transparency, higher-resolution patternability and higher heat resistance when being cured according to its use purpose. Therefore, makers conduct studies to prepare a forming material satisfying the aforementioned property requirements by selecting proper ingredients and determining a balanced formulation in production of the optical waveguide.
In the aforementioned R-to-R process for mass production, a dry film technique is generally employed in which the uncured film is used in a dry film form. In development of the material, requirements for process adaptability of the dry film material (e.g., lower tackiness and flexibility of the uncured film) consequently reduces the material design flexibility. In addition to the reduction in material design flexibility, the need for provision of lamination bases on opposite surfaces of the film is problematic for resource saving and costs in the formation of the dry film. Therefore, the adaptability to a wet process is also regarded as important in the development of the material (PTL 3).
In view of such technical background, a photosensitive epoxy resin composition satisfying the aforementioned property requirements has been developed, for example, by using a specific novolak polyfunctional epoxy resin as a base material and adding various resins to the epoxy resin (PTL 4).